Method for high speed equalization of packet data received from bus topology network, method for transmitting and receiving packet data in bus topology network, and receiver of bus topology network

ABSTRACT

A method of equalizing received packet data in a bus topology network, including: receiving, by a receiver of a second node, a first packet from a first node in a bus topology network in which two or more nodes are connected via a bus; setting, by the receiver, an equalizer coefficient of an equalizer using a first training sequence of the first packet and storing the set equalizer coefficient; receiving, by the receiver, a second packet including a second training sequence shorter than the first training sequence from the first node; and equalizing, by the receiver, the second packet using the stored equalizer coefficient.

TECHNICAL FIELD

The present invention relates to a method of transmitting and receivingpacket data in a bus topology network and a method of equalizing areceived packet.

BACKGROUND ART

Ethernet is a representative standard of wired bus topology packetnetworks. Ethernet based on up to 10 Mbps uses a bus topology network.However, when a data rate exceeds 100 Mbps, channel distortion becomessevere, and it is not possible to perform high-speed communicationwithout using an equalizer.

In a bus topology network, a plurality of receivers should be able toreceive a packet transmitted by a transmitter. However, since it is notpossible to know in advance to which receiver the transmitter transmitsa packet, equalizer training is difficult. Therefore, a network of 100Mbps or more uses a star topology network (star network). Since allEthernet nodes in a star network communicate with other network nodesthrough a switch, when a node transmits data, the correspondingreceivers are unified as a receiver in the switch, and only an equalizerfor a single transceiver link is required. Therefore, it is possible topreviously perform equalizer training for a channel between the node andthe switch, and high-speed data transmission can be performed on thebasis of the equalizer training.

DISCLOSURE Technical Problem

Although a star network is used to transmit a large amount of data, thestar network has a disadvantage in that a large number of switches andcables are required. For example, in systems such as cars, a bustopology network is usually used due to limited space and the weight ofcables.

The present invention is directed to providing a packet transmissionmethod which facilitates equalization of a receiver, a receiverincluding the corresponding equalizer, and a network system in atransceiver system on the basis of a bus topology network.

Technical Solution

One aspect of the present invention provides a method of equalizingpacket data at high speed, the method including: receiving, by areceiver of a second node, a first packet from a first node in a busincluding the first node and the second node; setting, by the receiver,an equalizer coefficient of an equalizer using a first training sequenceof the first packet and storing the set equalizer coefficient;receiving, by the receiver, a second packet including a second trainingsequence shorter than the first training sequence from the first node;and equalizing, by the receiver, the second packet using the storedequalizer coefficient.

The equalizing of the second packet may include: determining, by thereceiver, at least one of a sampling phase difference and a carrier wavephase difference between the first training sequence and the secondtraining sequence and correcting an input signal of the equalizer basedon the determined at least one of the sampling phase difference and thecarrier wave phase difference; and equalizing, by the equalizer to whichthe equalizer coefficient has been applied, the corrected input signal.

Another aspect of the present invention provides a method of receivingpacket data, the method including: receiving, by a receiver of areceiving node connected via a bus topology network, a packet from atransmitting node; determining, by the receiver, whether equalizertraining has been previously performed with a training packet previouslytransmitted by the transmitting node based on an identifier included inthe packet; when the equalizer training for the transmitting node hasbeen performed, loading, by the receiver, an equalizer coefficient forthe transmitting node to set an equalizer of the receiver; andequalizing, by the receiver, the packet using the equalizer having theequalizer coefficient.

The method may further include, when the equalizer training has beenperformed for the transmitting node, correcting, by the receiver, atleast one of a sampling phase difference and a carrier wave phasedifference which occurs when the equalizer equalizes the packet with theequalizer coefficient using the packet having a training sequenceshorter than a training sequence included in the training packet.

Another aspect of the present invention provides a receiver of a bustopology network, the receiver including: a coefficient extractiondevice configured to store an equalizer coefficient for at least onetransmitting node and extract an equalizer coefficient according to asource node of a received packet; a sampling device configured to samplea baseband signal in every symbol period; and an equalizer configured toequalize an output signal of the sampling device using the storedequalizer coefficient when the baseband signal is a signal received fromthe transmitting node.

The receiver of the bus topology network may further include a phasecorrector configured to correct a sampling phase difference and acarrier wave phase difference between an output of the equalizer and atraining sequence included in the baseband signal for the output signalusing the training sequence.

The phase corrector may determine an optimal sampling phase differenceand an optimal carrier wave phase difference for minimizing an error ofthe output of the equalizer among a plurality of quantized candidatesampling phase differences and carrier wave phase differences, firstlycorrect an input signal based on the determined optimal sampling phasedifference and the optimal carrier wave phase difference, determine acarrier wave phase difference value for minimizing an error between theoutput of the equalizer and each training symbol per training symbol,determine a final optimal carrier wave phase difference by adding anaverage of carrier wave phase differences determined for all symbols toa determined optimal carrier wave phase differences, and secondarilycorrect a phase of the corrected input signal based on the final optimalcarrier wave phase difference.

Another aspect of the present invention provides a method oftransmitting packet data, the method including: (a) transmitting, by atransmitter of a first node connected to a bus, a first packet in afirst format to a second node connected to the bus; (b) transmitting, bythe transmitter, a second packet in the first format to a third nodeconnected to the bus after step (a); (c) transmitting, by thetransmitter, a third packet in a second format to the second node afterstep (a); and (d) transmitting, by the transmitter, a fourth packet inthe second format to the third node after step (b). The first formatincludes a field of a first training sequence, and the second formatincludes a field of a second training sequence shorter than the firsttraining sequence.

The receiving nodes may set equalizer coefficients of the receivingnodes using the packets in the first format, and when the packets in thesecond format are received, the receiving nodes may correct at least oneof sampling phase differences and carrier wave phase differences betweenthe packets in the first format and the packets in the second format andequalize the packets in the second format using equalizers having theset equalizer coefficients.

Advantageous Effects

Technology described below makes it possible to relatively simplyconfigure a network using a bus topology network. Further, according tothe technology described below, a receiver identifies a transmittingnode which has transmitted a packet, and every time a packet isreceived, an equalizer is not trained again from the beginning butrapidly equalizes the received packet using a previous equalizersetting. Such a high-speed equalization technique reduces data overheadrequired for equalizer training and maximizes the throughput of asystem. Consequently, the technology described below enables high-speedpacket data communication on the basis of a bus topology network.

DESCRIPTION OF DRAWINGS

FIG. 1 is an example showing a process of generally transferring packetsin a bus topology network.

FIG. 2 is an example showing a process of transferring packets usinghigh-speed equalization in a bus topology network.

FIG. 3 is an example showing fields of packets used in FIG. 2.

FIG. 4 is an example of a flowchart of a method of receiving a packet ina bus topology network.

FIG. 5 is an example of a flowchart of a method of equalizing a receivedpacket at high speed in a bus topology network.

FIG. 6 is an example of a block diagram showing a configuration of areceiver 300 of a bus topology network.

FIG. 7 is an example of graphs illustrating a process of a samplingdevice when a training packet P1 and a data packet P3 are received fromthe same transmitting node.

FIG. 8 is an example of a block diagram showing a configuration of anequalizer in a receiver of a bus topology network.

FIG. 9 is an example of the complex plane for calculating an optimalcarrier wave phase.

MODES OF THE INVENTION

The technology described below may be variously modified and may havevarious exemplary embodiments, and specific exemplary embodiments willbe shown as examples in the drawings and described in detail. However,it is noted that the technology described below is not limited to thespecific exemplary embodiments but includes all possible modifications,equivalents and replacements which fall within the spirit and scope ofthe technology described below.

The terms, such as first, second, A, B, etc., may be used in describingvarious components, but the components are not limited by the terms. Theterms are used only to distinguish one component from other components.For example, a first component may be named a second component, and asecond component may be named a first component in a similar way withoutdeparting from the scope of the technology described below. The term“and/or” includes a combination or any of a plurality of associatedlisted items.

A singular expression includes a plural expression unless it is clearlyconstrued in a different way in the context. The terms used herein, suchas “including” and the like are used only to designate the features,numbers, steps, operations, components, parts, or combinations thereofdescribed in the specification, but should be construed not to excludepresence or addition of one or more other features, numbers, steps,operations, components, parts, or combinations thereof.

Prior to describing drawings in detail, a division of the configurationunits in the present specification is only by the main function of eachconfiguration unit. In other words, two or more of the configurationunits to be described below may be combined into a single configurationunit, or one configuration unit may be divided into two or more unitsaccording to subdivided functions. Each of the configuration units to bedescribed below may additionally perform a part or all of the functionsamong functions set for other configuration units other than beingresponsible for the main function, and some of main functions taken byeach of the configuration units may be exclusively taken and performedby other configuration units. Therefore, the presence of theconfiguration units each described through the present specificationshould be functionally interpreted. For this reason, it is clearly notedthat the configurations of configuration units of a receiver 300 and anequalizer 400 of a bus topology network according to the technologydescribed below can differ from the corresponding drawing within thelimits of achieving the purpose of the technology described below.

When a method or an operating method is performed, steps of the methodmay be performed in a different order from a described order unless aspecific order is clearly mentioned in the context. In other words,steps may be performed in the same order as described, performedsubstantially simultaneously, or performed in reverse order.

Hereinafter, a method of transmitting a packet in a bus topologynetwork, a method of receiving a packet in a bus topology network, amethod of equalizing a packet at high speed in a bus topology network, areceiver in a bus topology network and an equalizer of a bus topologynetwork receiver will be described in detail with reference to thedrawings.

FIG. 1 is an example showing a process 50 of generally transferringpackets in a bus topology network. In FIG. 1, it is assumed that a nodeA, a node B and a node C are connected via a bus and packets aretransmitted by the nodes at different times. FIG. 1 shows a case inwhich the node A and the node C transmit packets to the node B.

In FIG. 1, each node transmits a packet including a training sequenceand data. A packet transmitted by a transmitting node is generallysubjected to inter-symbol interference while passing through a channel.Therefore, a receiving node compensates for distortion caused by theinter-symbol interference using an equalizer of a receiver. In thefollowing description, it is assumed that an equalizer is configuredwith a feed-forward equalizer (FFE) which is a linear equalizer and afeedback equalizer (FBE) which is a non-linear decision-feedbackequalizer.

When complete information of a channel in which inter-symbolinterference occurs is obtained, it is possible to calculate anequalizer coefficient which corresponds to an inverse of the channel onthe basis of the information. However, in general, since a receiver doesnot have information of a channel, a transmitter of a transmitting nodeinitially transmits a training sequence that the receiver knows, and thereceiver trains an equalizer using the received training sequence anddetermines an equalizer coefficient.

An equalizer performs equalizer training using a training sequence of areceived packet to compensate for inter-symbol interference. Theequalizer determines an FFE coefficient and an FBE coefficient throughthe equalizer training and is set with the equalizer coefficients.Subsequently, the equalizer equalizes data included in the packet usingthe set equalizer coefficient. An output of the equalizer which has beencompensated for channel distortion is determined to be a value obtainedby adding an output of an FFE and a negative value of an output of anFBE.

Even if there is no specific description below, it is assumed that aprocess performed by a transmitting node is a process performed by atransmitter of the transmitting node and a process performed by areceiving node is a process performed by a receiver of the receivingnode.

In FIG. 1, first, the node A transmits a packet to the node B (51). Thepacket transmitted by the node A has a training sequence in a forepartthereof and data behind the training sequence. A receiver of the node Btrains an equalizer using the training sequence, compensates forinter-symbol interference of the following data using the trainedequalizer, and receives the data.

Subsequently, the node C transmits a packet to the node B (52). Thepacket transmitted by the node C also has a training sequence and data.Since different transmitting nodes have different characteristics ofwired channels in which packets are transmitted, the node B shouldperform equalizer training again in the receiver. Therefore, the node Bdiscards the equalizer coefficient optimized for a node A-node B linkand performs training for a node C-node B link.

Subsequently, the node A transmits a packet to the node B (53). Sincethe node B has just set the equalizer using the training sequence of thepacket transmitted by the node C (52), the node B should train theequalizer again using a training sequence transmitted by the node A.

Subsequently, the node A transmits another packet to the node B again(54). The node B has just received the packet from the node A but shouldtrain the equalizer again using a training sequence included in theother packet. This is because the node B, which is the receiving node,does not know which node is currently transmitting the other packet.Further, even when the node B knows that the same node is transmittingthe other packet, the equalizer of the node B loses time and phasesynchronization during a period in which there is no data exchange aftertransmission of the last packet, and thus the node B should performequalizer training again. In this way, a training sequence for trainingan equalizer is transmitted in each packet in a conventional bustopology network, and a receiver can receive the data after training theequalizer using the training sequence. As described above, in the bustopology network of FIG. 1, since a training sequence required forequalizer training should be carried every time a packet is transmitted,data transceiving efficiency notably deteriorates. Further, a techniquesuch as least mean square (LMS) is widely used as a training method foran equalizer coefficient, but the LMS technique requires a long time totrain for an equalizer coefficient. Therefore, time overhead for areceiver to perform equalizer training using a training sequence mayremarkably increase.

FIG. 2 is an example showing a process 100 of transferring packets usinghigh-speed equalization in the same bus topology network as in FIG. 1.FIG. 2 illustrates a case in which there are three nodes as an example,but it can be easily seen that the process 100 can be also applied to anarbitrary network in which two or more nodes are connected. Packets ofFIG. 2 transmitted by nodes have different structures than packets ofFIG. 1. First, structures of the packets transmitted by nodes in FIG. 2will be described. FIG. 3 is an example showing fields of the packetsused in FIG. 2. In FIG. 2, a node A transmits a packet P1 and a packetP3 to a node B, and a node C transmits a packet P2 and a packet P4.

All the packets include a preamble field and a packet type field incommon. Also, all the packets include a training sequence field incommon which varies in length according to packet types. The packets mayinclude a data field or may not. Since a data packet is a packet fortransmitting data, the data packet necessarily includes a data field,which will be described below.

A preamble field includes an identity (ID) of a transmitting node whichtransmits the corresponding packet and an ID of a receiving node whichreceives the packet. Packet types include a training packet (whosepacket type value is shown to be “0” in FIG. 3) and a data packet (whosepacket type value is shown to be “1” in FIG. 3).

A training packet is a packet for training an equalizer of a receivingnode. A receiver of the receiving node trains the equalizer using atraining sequence included in the training packet. The training sequenceincluded in the training packet is referred to as a first trainingsequence. The receiving node determines equalizer coefficients usingfirst training sequences and stores the determined equalizercoefficients according to transmitting nodes.

A data packet is a packet for transferring data to a receiving node. Adata packet also includes a training sequence, but the training sequenceis shorter than a first training sequence of a training packet. Thetraining sequence included in the data packet is referred to as a secondtraining sequence. The receiving node performs high-speed equalizationusing the second training sequence. A detailed process of high-speedequalization will be described below.

Packet transmission of FIG. 2 will be described according to time flow.In FIG. 2, the node A transmits the packet P1 to the node B (111). Thepacket P1 corresponds to a training packet. The node B identifieswhether the packet P1 is a training packet through a packet type anddetermines an equalizer coefficient using a training sequence (a firsttraining sequence) included in the training packet. When equalizertraining is successfully finished, the node B stores the equalizercoefficient for the node A which is a transmitting node in apredetermined storage device in a receiver and notifies the node A whichis the transmitting node that an equalizer has converged.

Subsequently, the node C transmits the packet P2 which is a trainingpacket to the node B (112). The node B determines that the receivedpacket P2 is a training packet and trains the equalizer using a trainingsequence (a first training sequence) included in P2. When equalizertraining is successfully finished, the node B stores an equalizercoefficient for the node C which is a transmitting node in apredetermined storage device in the receiver and notifies the node Cwhich is the transmitting node that the equalizer has converged.

Although not shown in FIG. 2, when equalizer training has not beenappropriately performed, the node B which is the receiving node mayrequest a transmitting node to transmit a training packet again.

As shown in FIGS. 2 and 3, the training packet P2 transmitted by thenode C includes a data field. According to how a protocol correspondingto communication rules and regulations is defined, a data field may beincluded in a training packet. The node B may train the equalizer usingthe training sequence included in the training packet P2, set anequalizer coefficient, and then continuously receive data included inP2.

Subsequently, the node A transmits the packet P3 which is a data packetto the node B again (113). The node B determines that P3 is a datapacket on the basis of packet type information. Subsequently, the node Bloads the equalizer coefficient for the node A from the internal storagedevice to set the equalizer, rapidly makes an equalizer setting using asecond training sequence, and then receives data in P3.

When there is not a change, such as an addition of a new node or aremoval of a node, in a bus and a physical state of the bus is notchanged, an inter-symbol interference characteristic of a wired channelwill not be changed. Therefore, it can be seen that the equalizercoefficient for which training has been performed using the packet P1will be similar to an equalizer coefficient appropriate for receivingthe packet P3 in the same node A-node B link. However, a point in timeat which the node B receives the packet P1 and a point in time at whichthe node B receives the packet P3 are different, and a phase of ademodulating carrier wave may be changed. Therefore, it is preferablefor the node B which is the receiving node to make a predeterminedmodification to the setting of the receiver. When receiving the packetP3, the receiver samples the signal at predetermined time intervals andtransfers the sampled signal to the equalizer. Here, since a phase atwhich sampling is performed is not the same as a phase at which thepacket P1 is received, a difference between reception times of the twopackets is expressed as a sampling phase difference. To compensate forsuch a sampling phase difference and a carrier wave phase difference, adata packet requires a predetermined training sequence. The receivermakes a setting for reusing a previous equalizer coefficient using thetraining sequence. The present technology describes a case in which asampling phase difference and a carrier wave phase difference arecorrected at a front end of an equalizer, but those of ordinary skill inthe art can easily understand that the same effect can be obtained bymaking a modification equivalent to such phase corrections to a part ofa receiver, for example, an equalizer coefficient. A training sequenceincluded in a data packet (a second training sequence) may be configuredto be much shorter than a training sequence included in a trainingpacket (a first training sequence). According to required performance ofa system, a length of a second training sequence may be set to be oneseveral tenth or one several hundredth the length of a first trainingsequence. As a result, it is possible to improve the throughput of asystem by reducing the number of training sequences and extending a datasection in a data packet.

Finally, the node C transmits the packet P4 which is a data packet tothe node B (114). The node B loads the equalizer coefficient for thenode C and corrects a sampling phase difference and a carrier wave phasedifference to perform high-speed equalization on the packet P4.

Operation of a transmitting node is additionally described. Atransmitting node should manage information on an equalizer trainingstate of a receiving node to which a generated packet will betransmitted. In FIG. 2, when an equalizer coefficient is successfullydetermined using the first training sequence of the training packet P1,the node B notifies the node A which is the transmitting node that anequalizer coefficient has been determined. The node A stores informationon the state in which an equalizer coefficient has been determined(equalization training has been performed normally) in the node B.

Subsequently, when the node A transmits a packet to the node B again,the node A confirms that equalization training has been completed in thenode B using the equalizer training state information thereof.Subsequently, the node A configures a data packet rather than a trainingpacket and transmits data. When the node A transmits data to a fourthnode in which an equalizer has not been set, the node A shouldpreviously set the equalizer in a receiver of the corresponding node bytransmitting a training packet to the node. After the corresponding nodenotifies the node A that an equalizer setting has been completed, thenode A transmits a data packet to the corresponding node.

FIG. 4 is an example of a flowchart of a method 200 of receiving apacket in a bus topology network.

In the method 200 of receiving a packet in a bus topology network, it isassumed that a transmitting node has transmitted a predetermined packet.As described above, the packet which has been transmitted by thetransmitting node includes a preamble field, a packet type field, and atraining sequence field.

The preamble field includes a start signal, a transmitting node ID, anda receiving node ID of the packet. The transmitting node ID and thereceiving node ID may be generally indicated as a combination of 0and 1. For example, the node A may be indicated by 0101, and the node Bmay be indicated by 1010.

First, a receiving node receives the packet transmitted by thetransmitting node (210). Actually, in a bus topology network, a packettransmitted by a specific transmitting node can be transferred to allreceiving nodes connected via a bus.

The receiving node which receives the packet checks the receiving nodeID of a preamble to determine whether the packet is intended to betransferred to the receiving node (220). For example, when the receivingnode is the node A (0101) and the receiving node ID included in thepreamble is “1010,” the node A does not process and ignores the receivedpacket (No of step 220). When the receiving node is the node B (1010)and the receiving node ID included in the preamble is “1010,” the node Bdetermines that a destination of the currently received packet is thenode B and processes the received packet (Yes of step 220).

Subsequently, the receiving node checks the packet type field of thepacket to determine a packet type (230). For consistency with FIGS. 2and 3, a training packet is assumed to have a packet type of 0. When thepacket type of the currently received packet is a training packet (Yesof step 230), the receiving node performs general equalizer trainingusing a first training sequence included in the packet (240). When datafollows the first training sequence like in P3 of FIG. 3, the receivingnode receives the data using an equalizer setting for which training hasbeen performed.

When the packet type of the received packet is not a training packet buta data packet (No of step 230), the receiving node loads an equalizercoefficient for the corresponding transmitting node on the basis of thetransmitting node ID. Subsequently, the receiving node equalizes thereceived packet at high speed using the loaded equalizer coefficient(250). While the packet is received, the coefficient of an equalizer maybe updated in a decision-feedback manner, which is intended for theequalizer to track a very slow change in channel.

After performing equalization, the receiving node calculates asignal-to-noise ratio (SNR) of an output of the equalizer (260). Evenafter the general equalizer training with the training packet (240) isfinished, the receiving node calculates an SNR of an output of theequalizer.

It is determined whether the SNR value is greater than a reference valueSNR_(th) (270). When the SNR value is greater than the reference valueSNR_(th), the receiving node determines that the equalizer hasconverged, stores the updated equalizer coefficient in an internalmemory, and notifies the transmitting node of a converged state of theequalizer (290). During this process, the receiving node may separatelystore the calculated SNR value and transfer the calculated SNR value tothe transmitting node.

When the SNR is smaller than SNR_(th), the receiving node determinesthat the equalizer has not converged (has diverged), discards thecurrent equalizer coefficient, and requests equalizer retraining fromthe transmitting node (280). In this case, the transmitting nodetransmits a training packet again, and transmits a data packet againafter the receiving node finishes equalizer training normally.

SNR_(th) is the reference value for determining whether an equalizer hasappropriately converged. As SNR_(th), different values may be usedaccording to a modulation technique, performance of a bus topologynetwork, a communication environment, and the like.

The process in which the receiving node performs high-speed equalization(250) in FIG. 4 will be described in detail below. FIG. 5 is an exampleof a flowchart of a method 250 of equalizing a received packet at highspeed in a bus topology network.

The receiving node is retaining an equalizer coefficient for a specifictransmitting node through equalizer training which has been previouslyperformed for the specific transmitting node. This process is performedthrough step 240 of FIG. 4.

Since a sampling phase or a carrier wave phase of a receiver may varybetween a point in time at which the receiving node receives a trainingpacket from the transmitting node and a point in time at which thereceiving node receives a data packet from the transmitting node, it isnecessary to correct the difference.

The receiver receives a data packet transmitted from the transmittingnode (251). The receiver loads an equalizer coefficient corresponding toa transmitting node ID included in the packet among stored equalizercoefficients (252).

The receiver calculates an optimal sampling phase difference and carrierwave phase difference using a training sequence (a second trainingsequence) included in the data packet and the loaded equalizercoefficient and makes a necessary correction. Specifically, the receiverdetermines an optimal sampling phase difference and carrier wave phasedifference using the second training sequence (253) and corrects anoutput signal of a sampling device of the receiver on the basis of thedetermined optimal sampling phase difference and carrier wave phasedifference (254). Subsequently, the receiver equalizes the packet usingthe corrected signal as an input of the equalizer (2550). Theequalization process will be described in further detail below withreference the receiver 400 of FIG. 8.

FIG. 6 is an example of a block diagram showing a configuration of areceiver 300 of a bus topology network.

In a channel, a signal transmitted by a transmitter of a transmittingnode is subjected to inter-symbol interference due to a characteristicaccording to a frequency, contaminated with white noise, and transferredto the receiver 300 of a receiving node.

The receiver 300 converts the received signal into a baseband through ademodulator 310 in the case of passband modulation, passes the receivedsignal through a receiving filter 320 as it is in the case of basebandconversion, and applies the signal to a sampling device 330. In the caseof passband modulation, a modulator of the transmitter and thedemodulator 310 of the receiver have an identical operating frequency f.However, since phase information of the transmitter is not known to thereceiver, there is a carrier wave phase difference of 0.

From now, it is assumed that a signal received by the receiver 300 is apassband signal. However, when a signal received by the receiver 300 isa baseband signal, no carrier wave is used, and there can be a samplingphase difference alone. In this case, the receiver 300 is required tocorrect only the sampling phase difference.

FIG. 7 is an example of graphs illustrating a process of a samplingdevice processing packets received from the same transmitting node.

FIG. 7 shows an analog signal sampling process in a sampling device of areceiver when the node B receives a first training sequence of thepacket P1 and then receives a training sequence of the packet P3 in thepacket transmission and reception process of FIG. 2. It is assumed thata received analog signal r₁ (t₁) of the packet P1 is sampled by thereceiver with an offset of an arbitrary value δ₁ on the basis of t=0which is a peak point of a first training signal.

When a length of a symbol period of received data is T, the samplingdevice samples the received signal r₁ (t₁) in every symbol period andtransfers the sampled signal to an equalizer. An output signal r_(n,1)of the sampling device at a time t=nT+δ₁ may be expressed by Equation 1below.

$\begin{matrix}{{r_{n,1} = {r_{1}\left( {{nT} + \delta_{1}} \right)}},{{- \frac{T}{2}} \leq \delta_{1} \leq \frac{T}{2}},{n = 0},1,2,\ldots} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In FIG. 7, δ₁ is a negative number. A first sample r_(0,1) correspondingto n=0, a second sample r_(1,1) corresponding to n=1, a third sampler_(2,1) corresponding to n=2, and a fourth sample r_(3,1) correspondingto n=3 are respectively sampled at t₁=δ₁, t₁=T+δ₁, t₁=2T+δ₁, andt₁=3T+δ₁ in sequence and transferred to the equalizer. In FIG. 7, it isassumed that first four bits of the training sequence transmitted in thepacket P1 are +1, −1, +1 and +1. The equalizer of the receiver istrained using all of the four bits and the following training data, andthe corresponding coefficient is stored in a memory of the equalizer.

Next, it is assumed that when an analog signal r₂ (t₂) of the trainingsequence of the packet P3 is input to the receiver as shown in FIG. 7,the receiver samples the analog signal r₂ (t₂) with an arbitrarysampling offset 82 on the basis of t₂=0 which is a peak point of a firsttraining signal of the packet P3 unlike the case of receiving the packetP1. It is assumed that first four bits of the training sequence of thepacket P3 are +1, −1, +1 and −1. Here, a sampled discrete signal r_(n,2)is expressed by Equation 2 below.

r _(n,2) =r ₂(nT+δ ₂),−T/2≦δ₂ <T/2[Equation 2]

In FIG. 7, δ₂ is assumed to be a positive number. Therefore, when thepacket P1 and the packet P3 are received, an output of the samplingdevice may have a sampling phase difference δ_(d) obtained bysubtracting δ₁ from δ₂ (δ_(d)=δ₂−δ₁), and the receiving node does notpreviously know the sampling phase difference. δ_(d) is a value greaterthan −T and smaller than T.

Therefore, to use an equalizer coefficient for which training has beenperformed upon receiving the packet P1 as it is when receiving thepacket P3 due to no change in the link between a time when receiving thepacket P1 and a time when receiving the packet P3, it is necessary totransfer a discrete signal obtained by compensating for the samplingphase difference δ_(d) and sampling the packet P3 again as an input ofthe equalizer. In other words, it is necessary to sample the analogsignal of the packet P3 using a delay value δ₁ of the packet P1 andtransfer the sampled signal to the equalizer. Further, there is acarrier wave phase difference of θ_(d) between the packet P1 and thepacket P3, but the receiving node does not know the carrier wave phasedifference either.

When the receiver accurately estimates and compensates for δ_(d) andθ_(d), it is possible to directly perform data receiving using theprevious equalizer coefficient without additional equalizer training.

FIG. 8 is an example of a block diagram showing a configuration of anequalizer 400 in a receiver of a bus topology network. FIG. 8 shows aconfiguration corresponding to an equalizer 340 of FIG. 6.

The equalizer 400 of a bus topology network includes a coefficientextraction device 410 which stores an equalizer coefficient for at leastone transmitting node and extracts an equalizer coefficient according toa source node of a received packet, and an equalizer 430 which equalizesan output signal of a sampling device using a stored equalizercoefficient. In addition, the equalizer 400 of a bus topology networkmay further include a phase corrector 420 which corrects a samplingphase difference and a carrier wave phase difference between an outputof the equalizer and a training sequence included in a baseband signalusing the training sequence for an output signal of the sampling devicethat samples the baseband signal in every symbol period.

First, when a data packet is received, the receiver interprets apreamble to check whether the data packet is intended to be transferredto the receiver. When receiver IDs are different, the receiver waitsuntil a new packet is received. When the receiver IDs are identical andthe received packet is a training packet, the receiver performsequalizer training. When the received packet is a data packet, thereceiver calculates δ_(d) and θ_(d) using a second training sequence inthe packet to correct a signal output from the sampling device 330,transfers the corrected signal to the equalizer, and proceeds to a stepof receiving data.

When the received packet is a data packet and an equalizer coefficientfor the corresponding transmitting node has been stored, the equalizerextracts an equalizer coefficient for the transmitting node of thepacket from a storage unit 413 and stores the equalizer coefficient ineach of internal memories of an FFE 431 and an FBE 432. It is assumedthat the FFE 431 has M internal memories and the FBE 432 has N internalmemories.

The equalization unit 430 includes the FFE 431, the FBE 432, an adder433, and a determiner 434.

Output signals of the sampling device are sequentially input to the Mmemories in the FFE 431. A current symbol value and previously receivedM−1 signals are respectively multiplied by FFE coefficients w₀, w₁, . .. , and w_(M), and the products are summed up by the adder to obtain anoutput F_(n) of the FFE. The FBE 432 inputs a determination value ŝ_(n)which has passed through the determiner 434 to the N memories therein,multiplies outputs of the memories respectively by FBE coefficients b₁,b₂, . . . , and b_(N), and sums up the products through the adder tocalculate an output B_(n) of the FBE. An output of the overall equalizeris calculated by the adder 433 adding the output F_(n) of the FFE and anegative value of the output B_(n) of the FBE.

The coefficient extraction device 410 includes a storage unit 413 whichstores equalizer coefficients for which training has been performedusing training sequences included in packets received from transmittingnodes and IDs of the transmitting nodes, a transmitting nodeidentification unit 411 which extracts a transmitting node ID from areceived packet, and a coefficient selection unit 412 which selects anequalizer coefficient corresponding to the transmitting node ID in thestorage unit.

A process of correcting a sampling phase difference and a carrier wavephase difference of an output signal of a sampling device will bedescribed in detail below. The phase corrector 420 performs the processof correcting the sampling phase difference and the carrier wave phasedifference.

The phase corrector 420 includes a phase estimation unit 421 whichestimates a sampling phase difference and a carrier wave phasedifference, a sampling correction unit 422 which corrects a samplingphase of an output signal on the basis of a sampling phase difference,and a carrier wave correction unit 423 which corrects a carrier wavephase of the signal whose sampling phase has been corrected on the basisof a carrier wave phase difference.

The sampling correction unit 422 receives an input discrete signalr_(n,2) as an input and generates a discrete signal sequence r _(n,2)having an arbitrary time delay. In general, a timing conversion filteris widely used for sampling correction. A variety of methods ofgenerating a discrete signal having an arbitrary time delay with alinear sum of an input signal are known, and all the methods can beused. The sample r _(n,2) whose sampling phase has been corrected isthereafter provided as an input of the carrier wave correction unit 423.When a phase rotation value is θ, this device outputs a signal {tildeover (r)}_(n,2) whose phase has been rotated by θ by multiplying theinput by e^(jθ).

{tilde over (r)}_(n,2) is provided as an equalizer input. An equalizeroutput {tilde over (s)}_(n) obtained by subtracting the output B_(n) ofthe FBE from the output F_(n) of the FFE in the equalizer is expressedby Equation 3 below.

$\begin{matrix}{{\overset{\sim}{s}}_{n} = {{e^{j\; \theta}{\sum\limits_{k = 0}^{M - 1}{w_{k}{{\overset{\_}{r}}_{{n - k},2}(\delta)}}}} - {\sum\limits_{k = 1}^{N}{b_{k}{\hat{s}}_{n - k}}}}} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack\end{matrix}$

w_(k) is an FFE coefficient among equalizer coefficients, b_(k) is anFBE coefficient among equalizer coefficients, and ŝ_(n) is an output ofthe determiner 434 of the equalizer.

w_(k) and b_(k) are equalizer coefficients calculated using previoustraining packets. When K training sequences are transmitted in datapackets, an overall error Γ of the equalizer output {tilde over (s)}_(n)and training data s_(n) is given by Equation 4 below.

$\begin{matrix}{{\Gamma \left( {\delta,\theta} \right)} = {{\sum\limits_{n = 1}^{K}{{s_{n} - {\overset{\sim}{s}}_{n}}}^{2}} = {\sum\limits_{n = 1}^{K}{{s_{n} - {e^{j\; \theta}{\sum\limits_{k = 0}^{M - 1}{w_{k}{{\overset{\_}{r}}_{{n - k},2}(\delta)}}}} + {\sum\limits_{k = 1}^{N}{b_{k}{\hat{s}}_{n - k}}}}}^{2}}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

The phase estimation unit 421 of the phase corrector 420 calculates δand θ for minimizing Γ(δ, θ) of Equation 4. Values minimizing the errorare referred to as an optimal sampling phase difference {circumflex over(δ)}_(d) and an optimal carrier wave phase difference {circumflex over(θ)}_(d). This process determines the parameters {circumflex over(δ)}_(d) and {circumflex over (θ)}_(d) so that the output of theequalizer becomes as much similar as possible to training data.

Assuming that the equalizer converges and the output of the determinerhas no error, a substitution can be made as follows: ŝ_(n-k)=s_(n-k),and when b₀ is defined to be equal to 1, Γ(δ, θ) may be arranged asshown in Equation 5 below.

$\begin{matrix}{{\Gamma \left( {\delta,\theta} \right)} = {\sum\limits_{n = 1}^{K}{{{\sum\limits_{k = 0}^{N}{b_{k}s_{n - k}}} - {e^{j\; \theta}{\sum\limits_{k = 0}^{M - 1}{w_{k}{{\overset{\_}{r}}_{{n - k},2}(\delta)}}}}}}^{2}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

In the above equation, the first term Σk₌₀ ^(N)b_(k)s_(n-k) is a fixedvalue irrelevant to δ and θ and thus indicates Φ_(n), and the secondterm may be defined using the following equation:

F _(n)(δ)=Σ_(k=0) ^(M-1) w _(k) *r _(n-k,2)(δ).

Then, Equation 5 may be simplified as shown in Equation 6 below.

$\begin{matrix}{{\Gamma \left( {\delta,\theta} \right)} = {\sum\limits_{n = 1}^{K}{{\Phi_{n} - {e^{j\; \theta}{F_{n}(\delta)}}}}^{2}}} & \left\lbrack {{Equation}\mspace{14mu} 6} \right\rbrack\end{matrix}$

e^(jθ)F_(n)(δ) is an output of the FFE when a delay value of a timingconverter is set to δ and a carrier wave phase rotation value is set toθ. Therefore, Φ_(n) may be previously calculated using a known trainingsequence, and FFE outputs may be calculated from all possiblecombinations of δ and θ to find δ and θ for minimizing the difference.

In brief, an optimal sampling phase difference and an optimal carrierwave phase difference which result in the minimum error of an equalizeroutput are selected from among a plurality of candidate sampling phasedifferences and a plurality of candidate carrier wave phase differences.Subsequently, a signal input to the equalizer (an output signal of thesampling device) is corrected so that the input signal of the equalizermay have an optimal sampling phase and an optimal carrier wave phase.

However, since θ may have a continuous arbitrary value between 0 and 2πand δ may have an arbitrary value between −T and T, it is very difficultto calculate optimal estimated values for minimizing Γ(δ, θ) from allvalues of δ and θ.

Therefore, a more practical method involves calculating estimated valuesfor minimizing Γ(δ, θ) from a predetermined number of quantized valuesof δ and θ. In other words, the complexity of calculation is lowered byquantizing candidate sampling phase differences and candidate carrierwave phase differences. This includes two steps. First, in a first step,{tilde over (δ)} and θ which are optimal quantized values arecalculated, and in a second step, a more accurate value {circumflex over(θ)} is obtained through an additional calculation.

In the first step, values which result in the minimum error are foundamong quantized values of δ and θ.

First,

$\theta_{i} = {\frac{2\; \pi}{U}i}$

(i=0, . . . , and U−1) which is obtained by quantizing θ in U units of apredetermined angle between 0 degrees and 2π is used. When U has a largevalue, an error is calculated from a precise angle, and an accuratephase error value can be obtained, but there is a disadvantage of anincrease in the complexity.

Quantized

$\delta_{j} = {\left( {\frac{j}{V} - \frac{1}{2}} \right)T}$

(j=−V, . . . , and V−1) is used as a sampling phase difference.

$\Gamma \left( {{\frac{2\; \pi}{U}i},{\left( {\frac{j}{V} - \frac{1}{2}} \right)T}} \right)$

is calculated for a total of U×2V combinations, and δ_(j) and θ_(j) forminimizing

$\Gamma \left( {{\frac{2\; \pi}{U}i},{\left( {\frac{j}{V} - \frac{1}{2}} \right)T}} \right)$

are calculated and expressed as δ and θ. {tilde over (δ)} is determinedas the aforementioned optimal sampling phase difference {circumflex over(δ)}_(d). The sampling correction unit 422 corrects the signal using{circumflex over (δ)}_(d) and then corrects a carrier wave phasedifference more precisely in the second step.

In the second step, a more accurate carrier wave phase difference iscalculated on the basis of the determined optimal sampling phasedifference {circumflex over (δ)}_(d). A highly accurate carrier wavephase value is calculated because even a small carrier wave phasedifference directly leads to an error when a high-order modulationscheme, such as 64 quadrature amplitude modulation (64QAM), 256QAM, orthe like, is used. An improvement in the carrier wave phase differenceinvolves calculating a residual carrier wave phase difference on thebasis of the estimated value {tilde over (θ)} calculated in the firststep.

To this end, the phase is expressed as follows: θ=θ_(r)+{tilde over(θ)}. θ_(r) is a residual carrier wave phase difference to be estimatedin the second step. Here, the equalizer output error Γ(δ, θ) is arrangedas shown in Equation 7 below.

$\begin{matrix}{{\Gamma \left( {\overset{\sim}{\delta},{\theta_{r} + \overset{\sim}{\theta}}} \right)} = {\sum\limits_{n = 1}^{K}{{\Phi_{n} - {e^{j\; \theta_{r}}e^{j\; \overset{\sim}{\theta}}{F_{n}\left( \overset{\sim}{\delta} \right)}}}}^{2}}} & \left\lbrack {{Equation}\mspace{14mu} 7} \right\rbrack\end{matrix}$

Since calculation of θ_(r) minimizing Γ(δ, θ_(r)+{tilde over (θ)})requires a very complex calculation process, a simpler method isproposed.

A simplified technique of calculating θ_(r)(n), which is a value ofθ_(r) minimizing an error of each training symbol s_(n), that is,∥Φ_(n)−e^(jθ) ^(r) e^(j{tilde over (θ)})F_(n)(δ)∥, separately accordingto symbols and averaging θ_(r)(n) (n=1, . . . , and K) without directlycalculating a value minimizing Γ({tilde over (δ)}, θ_(r)+{tilde over(θ)}) is used.

When {acute over (F)}_(n) is defined as follows: {tilde over(F)}_(n)=e^(j{tilde over (θ)})F_(n)({tilde over (δ)}), it can be easilyseen that it is optimal to set θ_(r) for minimizing the error betweenΦ_(n) and {tilde over (F)}_(n) to an angle between φ_(n) and {tilde over(F)}_(n) in the complex plane. FIG. 9 is an example of the complex planefor calculating an optimal phase.

An estimated residual phase value {dot over (θ)}_(r) is calculated byaveraging θ_(r)(n) (n=1, . . . , and K) which are calculated for Ktraining sequences as shown in Equation 8 below.

$\begin{matrix}{{\overset{.}{\theta}}_{r} = {\frac{1}{K}{\sum\limits_{n = 1}^{K}{\theta_{r}(n)}}}} & \left\lbrack {{Equation}\mspace{14mu} 8} \right\rbrack\end{matrix}$

A final carrier wave phase difference {circumflex over (θ)}_(d) is {dotover (θ)}_(r)+{tilde over (θ)}. The carrier wave correction unit 423rotates an output signal of the sampling correction unit 422 by theoptimal carrier wave phase difference {circumflex over (θ)}_(d) in thecomplex plane and transfers the rotated output signal as an equalizerinput. In this way, when the sampling phase difference and the carrierwave phase difference of the output signal of the sampling device arecorrected and the output signal is transferred as an equalizer input, itis possible to directly start data receiving using a previous equalizercoefficient without additional equalizer training.

Meanwhile, after the packet is received, the receiver may calculate anSNR of the equalizer output {tilde over (s)}_(n) using Equation 9 below.

$\begin{matrix}{{S\; N\; R} = \frac{\sum\limits_{n = 1}^{K}{{\overset{\sim}{s}}_{n}}^{2}}{\sum\limits_{n = 1}^{K}{{s_{n} - {\overset{\sim}{s}}_{n}}}^{2}}} & \left\lbrack {{Equation}\mspace{14mu} 9} \right\rbrack\end{matrix}$

Here, K is the number of training symbols for calculating the SNR. Whenthe SNR value is greater than the reference value SNR_(th), it isdetermined that the equalizer has converged, and when the SNR is smallerthan SNR_(th), it is determined that the equalizer has not converged,and a message is transmitted to the transmitting node to requestequalizer training again.

The present embodiment and the attached drawings are merely illustrativeto describe a part of the technical spirit included in theabove-described technology. Therefore, it is apparent that modificationsand specific embodiments which those of ordinary skill in the art caneasily infer within the scope of the technical spirit included in thespecification and drawings of the above-described technology fall withinthe technical scope of the above-described technology.

1. A method of equalizing received packet data in a bus topologynetwork, the method comprising: receiving, by a receiver of a secondnode, a first packet from a first node in a bus topology network inwhich two or more nodes are connected via a bus; setting, by thereceiver, an equalizer coefficient of an equalizer using a firsttraining sequence of the first packet and storing the set equalizercoefficient; receiving, by the receiver, a second packet including asecond training sequence shorter than the first training sequence fromthe first node; and equalizing, by the receiver, the second packet usingthe stored equalizer coefficient.
 2. The method of claim 1, wherein theequalizing of the second packet comprises determining, by the receiver,at least one of a sampling phase difference and a carrier wave phasedifference between the first packet and the second packet, changing areceiver setting based on the determined at least one of the samplingphase difference and the carrier wave phase difference, and equalizingthe second packet.
 3. (canceled)
 4. The method of claim 1, wherein theequalizing of the second packet comprises correcting an input signal ofthe equalizer based on at least one of a sampling phase difference and acarrier wave phase difference between the first training sequence andthe second training sequence.
 5. The method of claim 1, wherein theequalizing of the second packet comprises determining an optimalsampling phase difference and an optimal carrier wave phase differencefor minimizing an error of an output of the equalizer among a pluralityof quantized candidate sampling phase differences and carrier wave phasedifferences, firstly correcting the input signal based on the optimalsampling phase difference and the optimal carrier wave phase difference,determining a carrier wave phase difference value for minimizing anerror between the output of the equalizer and each training symbol pertraining symbol, determining a final optimal carrier wave phasedifference by adding an average of carrier wave phase differences of allsymbols to the determined optimal carrier wave phase difference, andsecondarily correcting a phase of the corrected input signal based onthe final optimal carrier wave phase difference.
 6. The method of claim5, wherein the optimal carrier wave phase difference θ and the optimalsampling phase difference δ are determined to be values for minimizingan equation Γ(δ, θ) below:${\Gamma \left( {\delta,\theta} \right)} = {{\sum\limits_{n = 1}^{K}{{s_{n} - {\overset{\sim}{s}}_{n}}}^{2}} = {\sum\limits_{n = 1}^{K}{{s_{n} - {e^{j\; \theta}{\sum\limits_{k = 0}^{M - 1}{w_{k}{{\overset{\_}{r}}_{{n - k},2}(\delta)}}}} + {\sum\limits_{k = 1}^{N}{b_{k}{\hat{s}}_{n - k}}}}}^{2}}}$(where K is a number of second training sequences, {tilde over (s)}_(n)is an equalizer output, s_(n) is a second training sequence, w_(k) is afeed-forward equalizer (FFE) coefficient among equalizer coefficients,b_(k) is a feedback equalizer (FBE) coefficient among the equalizercoefficients, and r _(n) is a signal output from a sampling device). 7.(canceled)
 8. The method of claim 1, wherein the first packet includesan identification information indicating that the first trainingsequence for setting the equalizer coefficient is included, and thesecond packet includes identification information indicating that thepreviously set equalizer coefficient is used.
 9. A method of receivingpacket data in a bus topology network, the method comprising: receiving,by a receiver of a receiving node connected via a bus topology network,a packet from a transmitting node; determining, by the receiver, whetherequalizer training has been previously performed with a training packetpreviously transmitted by the transmitting node based on an identifierincluded in the packet; when the equalizer training for the transmittingnode has been performed, loading, by the receiver, an equalizercoefficient for the transmitting node to set an equalizer of thereceiver; and equalizing, by the receiver, the packet using theequalizer having the equalizer coefficient, wherein the identifierincludes at least one of an identity (ID) of the transmitting node orinformation indicating whether an equalizer setting is required. 10.(canceled)
 11. (canceled)
 12. (canceled)
 13. (canceled)
 14. The methodof claim 9, further comprising, when a signal-to-noise ratio (SNR) of adeterminer output of the equalizer is smaller than a reference valueafter the packet is equalized, requesting, by the receiver, a trainingpacket for equalizer training from the transmitting node.
 15. The methodof claim 9, wherein the training packet includes i) a preamble includinga transmitting node identity (ID) and a receiving node ID, ii) packettype information, and iii) a first training sequence, and the packetincludes i) a preamble including the transmitting node ID and thereceiving node ID, ii) packet type information, iii) a second trainingsequence shorter than the first training sequence, and iv) data. 16.(canceled)
 17. A receiver of a bus topology network, comprising: acoefficient extraction device configured to store an equalizercoefficient for at least one transmitting node and extract an equalizercoefficient according to a source node of a received packet; a samplingdevice configured to sample a baseband signal in every symbol period;and an equalizer configured to equalize an output signal of the samplingdevice using the stored equalizer coefficient when the baseband signalis a signal received from the transmitting node.
 18. The receiver ofclaim 17, wherein the coefficient extraction device comprises: a storageunit configured to store the equalizer coefficient for which traininghas been performed using a training sequence included in a packetreceived from the transmitting node and an identity (ID) of thetransmitting node; a transmitting node identification unit configured toextract a transmitting node ID from the received packet; and acoefficient selection unit configured to select an equalizer coefficientcorresponding to the transmitting node ID in the storage unit.
 19. Thereceiver of claim 17, further comprising a phase corrector configured tocorrect a sampling phase difference and a carrier wave phase differencebetween an output of the equalizer and a training sequence included inthe baseband signal for the output signal using the training sequence.20. The receiver of claim 19, wherein the phase corrector comprises: aphase estimation unit configured to estimate the sampling phasedifference and the carrier wave phase difference; a sampling correctionunit configured to correct a sampling phase of the output signal basedon the sampling phase difference; and a carrier wave correction unitconfigured to correct a carrier wave phase of the signal whose samplingphase has been corrected based on the carrier wave phase difference. 21.The receiver of claim 19, wherein the phase corrector determines anoptimal sampling phase difference and an optimal carrier wave phasedifference for minimizing an error of the output of the equalizer amonga plurality of quantized candidate sampling phase differences andcarrier wave phase differences, firstly corrects an input signal basedon the determined optimal sampling phase difference and the optimalcarrier wave phase difference, determines a carrier wave phasedifference value for minimizing an error between the output of theequalizer and each training symbol per training symbol, determines afinal optimal carrier wave phase difference by adding an average ofcarrier wave phase differences determined for all symbols to adetermined optimal carrier wave phase differences, and secondarilycorrects a phase of the corrected input signal based on the finaloptimal carrier wave phase difference.
 22. (canceled)
 23. The receiverof claim 17, wherein the equalizer determines the equalizer coefficientusing a training packet received from the transmitting node andtransfers the determined equalizer coefficient to the coefficientextraction device to store the determined equalizer coefficient. 24.(canceled)
 25. A method in which a transmitter of a transmitting nodeconnected to a bus topology network transmits a plurality of packets toa plurality of receiving nodes, the method comprising: (a) transmitting,by the transmitter of a first node connected to the bus, a first packetin a first format to a second node connected to the bus; (b)transmitting a second packet in the first format to a third nodeconnected to the bus after step (a); (c) transmitting, by thetransmitter, a third packet in a second format to the second node afterstep (a); and (d) transmitting, by the transmitter, a fourth packet inthe second format to the third node after step (b), wherein the firstformat includes a field of a first training sequence, and the secondformat includes a field of a second training sequence shorter than thefirst training sequence.
 26. The method of claim 25, wherein thereceiving nodes set equalizer coefficients of the receiving nodes usingthe packets in the first format, and when the packets in the secondformat are received, the receiving nodes correct at least one ofsampling phase differences and carrier wave phase differences betweenthe packets in the first format and the packets in the second format andequalize the packets in the second format using equalizers having theset equalizer coefficients.
 27. (canceled)
 28. (canceled)